korlov42 commented on code in PR #4992:
URL: https://github.com/apache/ignite-3/pull/4992#discussion_r1905364002
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modules/sql-engine/src/main/java/org/apache/ignite/internal/sql/engine/metadata/IgniteMdRowCount.java:
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@@ -100,68 +106,334 @@ public double getRowCount(IgniteLimit rel,
RelMetadataQuery mq) {
}
/**
- * JoinRowCount.
- * TODO Documentation https://issues.apache.org/jira/browse/IGNITE-15859
+ * Estimates the number of rows produced by a join operation.
+ *
+ * <p>This method calculates an estimated row count for a join by
analyzing the join type,
+ * join keys, and the cardinality of the left and right inputs. It
provides specialized
+ * handling for primary key and foreign key relationships. When certain
metadata is unavailable
+ * or when specific conditions are not met, it falls back to Calcite's
default implementation
+ * for estimating the row count.
+ *
+ * <p>Implementation details:</p>
+ * <ul>
+ * <li>If the join type is not {@link JoinRelType#INNER}, Calcite's
default implementation is used.</li>
+ * <li>If the join is non-equi join, Calcite's default implementation is
used.</li>
+ * <li>The row counts of the left and right inputs are retrieved using
+ * {@link RelMetadataQuery#getRowCount}. If either value is unavailable,
the result is {@code null}.</li>
+ * <li>If the row counts are very small (≤ 1.0), the method uses the
maximum row count as a fallback.</li>
Review Comment:
this is inlined fallback to Calcite's default implementation, see
`org.apache.calcite.rel.metadata.RelMdUtil#getJoinRowCount`
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